This 5-year R01, competing continuation study will examine links between the recent legalization of cannabis for adults (age 21+) in Washington State and patterns of cannabis use and related risk behaviors, including other substance use, conduct problems, and HIV sexual risk behavior (SRB) among youth. It also will examine links between cannabis legalization for adults and changes in antecedents of youth cannabis use, including youth norms; parent cannabis and other substance use and norms; parenting practices; perceived peer, sibling, and partner use and norms; and availability. Analyses will test whether any changes in youth cannabis use and associated risk behavior are explained (mediated) by changes in antecedents of youth cannabis use. The proposed project is uniquely suited to address these goals. It builds on The Intergenerational Project (TIP), a study that uses an accelerated longitudinal design, and aims to understand the effects of parent substance use on child development (n = 383 families, 80% living in Washington State). TIP includes parents drawn from a prior longitudinal panel study (the Seattle Social Development Project (SSDP)), their oldest biological child, and a second caregiver when available. TIP collected recent (2010) pre-legalization data, setting the stage for an evaluation of this important policy shift. Availabl pre-legalization youth data span ages 1-22 and parent data span ages 27-35 (2002-2010, 7 assessments). The proposed study would add 3 additional annual data collections post-legalization in 2015, 2016, and 2017 with parents, youth, and second caregivers. The proposed project is driven by the Social Development Model and Problem Behavior Theory. The effects on youth of legalized adult cannabis use are unknown. This study will be the first to test links between cannabis legalization for adults and youth cannabis use, associated risk behavior, and their antecedents. The accelerated longitudinal design will enable us to use overlapping cohorts to map trajectories of youth outcomes pre- and post-legalization, disentangling development and history (policy change) and facilitating understanding about the intersection of cannabis legalization and development in ways that other studies cannot. We are aware of no other ongoing accelerated longitudinal study of youth in either Washington or Colorado, making TIP uniquely poised to examine the impact of cannabis legalization on youth in a developmentally informed way. Results will provide information critical to inform state and national cannabis policy and prevention of youth cannabis use in the context of legalization.